Electrical network



ELECTRICAL NETWORK Filed Nov. 17, 1942 i 5 Sheets-Sheet l I 4 h E |8 3 5 4 FIG-6 INVEN'I'OR EDWIN K. ST DOLA BY ATTORNEY ELEcTfiIcAL NETWORK,

Filed Nov. 17, 1942 5 Sheets-Sheet? FIG'Z IN VENTOR Eowgfi K. STODOLA BY $442 ATTORNEY Apnl 9, 1946. E. K. STODOLA 2,397,992

ELECTRICAL NETWORK Filed Nov. 17, 1942. 5 Sheets-Sheet 5 FIG-4 GRID VOLTAGE INVENTOR Eowm K. 5T0 OLA A TTORNE Y Apnl 9, 1946. E. K. STODQLA 2,397,992

1 ELECTRICAL NETWORK Filed Nov. 17, 1942 5 Sheets-Sheet 4 FIG-9 Mir/0 15/: 0.6

7 x= ISOOO WW5 i 9/7614: an

X $000 Of/MS GRID VOL TAGE INVENTOR Eowm K. STODOLA W fl/ A TTORNE Y TO OSCILLATOR FIG. IO

E. K. STODOLA ELECTRICAL NETWORK Filed Nov. 17, 1942 FIG. II

INDUCTIVE LIMITS OF LINEAR OPERATIONS OF T E'OONTROLLED VOL'IAGE'DIVIDER 4 0.77 0462 0.4 REhATNE LENGTHHOD INVENTOR EDWIN K. ST MR M449. we!

DOLA

ATTORNEY Patented Apr. 9, 1946 Application November 17, 1942, Serial No. 465,921

13 Claims. (01. 179-1715) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 75'!) 1 a The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates to electrical networks, particularly those which are suitable for modulation of carrier waves.

One object of my invention is to provide a resistance or impedance network, that may be controlled to function as a voltage-divider by modulating an electron tube, or other non-linear impedance, in the network, so that the output potential between two predetermined points of the network will vary as a linear function of the modulating force applied to the electron tube or have a non-linear function diflerent from that of the impedance. 4

Another object of my invention is to provide a resistance or impedance network including an electron tube, with the characteristics of the network elements so related to the characteristics of the electron tube, that modulation of the tube by an external force will provide a resultant output voltage, between two predetermined points of the network, that shall vary as a linear function of the modulating force that is applied to the tube.

- Another object of my invention is to provide an impedance network, including an electron tube, with the characteristics of the network so related to the characteristics of the tube, that the phase angle of an output voltage between two predetermined points of the network may be controlled and caused to vary substantially as a linear function of a modulating force applied to the electron tube. 1

Another object of my invention is to provide a phase shifting network, including an electron function of the grid voltage. so the volta e between two predetermined points of the network will be a linear function of the effective voltage on the grid of the electrontube.

- In a radio system in which a carrier voltage is to be controlled to establish phase or frequency modulation, it is desirable to establish a direct or linear relationship between the modulating control voltage and the resulting modulation effect. a

To establish such-relationship I provide a voltage dividing network, from which an output voltage may be derived that is a direct linear functube, with the characteristics of the network so related to the characteristics of the tube that the output voltage of the network may be shifted anguiarly with respect to a basic applied voltage, by the effect of, and in proportion to the amplitude of, an external modulating control voltage applied to the tube.

Another object of my invention. is to provide a -modulation system, wherein a carrier voltage is tion of a modulating control voltage that is applied to an electron control tube in the network. The plate-cathode im edance of a standard commercial triode, and hence the voltage-drop across the triode, does not vary in direct proportion to the varying potential of the grid. In order to procure a voltage drop across the tube, that shall be relatively a linear function of the grid voltage, I provide a resistor or impedance element, of proper characteristics and constants, in series with the tube to constitute a voltage dividing circuit, across .which a fixed voltage, such as the carrier voltage, may be applied. With a resistor of proper value connected in series with the triode, when the plate-cathode impedance is varied by varying the grid voltage, the ratio 'of the plate-cathode voltage drop to the total voltage drop across the circuit, including the tube and the fixed resistor, may be made to follow substantially as a direct or linear function of the modulating voltage applied to the grid of the tube through a substantial partof the relationship.

The voltage drop across the tube, in such voltage dividing circuit, may then be utilized as a control voltage that bears a direct or linear relationship to the modulating voltage which is applied The voltage dividing circuit may thus be employed to provide a plate-cathode voltage drop, the value of which is a substantially direct or linear function of the modulating voltage appliedto the grid, or to provide a voltage that is phasedisplaced from a base voltage, in accordance with the efl'ective excitation of the grid of the tube by the external modulating voltage.

The manner in whichthe voltage dividing cir cult is employed to establish such phase or frequency modulation is illustrated in the accommarket;

Figure 3 is a diagram of a voltage dividing circult, in which a resistor is connected in a series with a triode, to establish a direct linear proportionality between the voltage drop across the triode and the total applied voltage across the triode and the resistor;

Figure 4 is a family of curves, showing the relationship between the grid-voltage and the ratio between the tube voltage drop and total applied voltage across the tube and resistor;

Figure 5 is an equivalent diagram of the voltage dividing circuit, with a reactor in series with the tube;

Figure 6 is a diagram of a circuit showing the tube' and the reactor in series to constitute a voltage dividing circuit:

Figure '7 is voltage vector triangle for the voltage dividing clrcult of Fig. 6 when the reactor is inductive;

Figure -8 is a similar triangle of the voltages when the reactor is capacitive;

Figure 9 is a graph showing a family of curves directed to the relationship of the phase angle between the voltage across the tube and the total applied voltage, as controlled by the excitation of the grid, in the circuit of Fig. 6;

Figure 10 is a simple schematic diagram of a circuit network for establishing a voltage triangle voltage dividing circuit, is kept constant, and the resistors varied, the voltage drop across the two resistors 2 and 3 will vary according to the values of those resistors. The ratio of the output voltage of circuit 5 to the input voltage of circuit 4 will vary according to the ratio of the resistance of resistor 3 to the sum of the resistors 2 and 3.

Where the resistance 2 is kept at constant value and the resistor 3 is made variable, the drop in potential across resistor 3 will not vary directly in proportion to the change of resistance of the resistor 3, but will vary instead according to the ratio of' the resistor 3 to the total resistance of the voltage dividing circuit.

, pedance to the impressed frequency.

of particular construction, whereirom a voltage is derived to energize a voltage dividing circuit; Figure 11 is a voltage diagram-oi a symmetrical layout as in Fig. 10;

Figure 12 shows a non-symmetrical voltage triangle of the system of Fig. 13; and

' Figure 13 is a detailed circuit diagram of a phase modulation system embodying the features of Fig. 10.

In Fig. 2 is illustrated a simple graph i, showing the relationship between the excitation voltage on the grid of an electron tube, such as a triodaand the resistance of the cathode to plate of the tube.

In the present invention, the electron tube is utilized as a variable resistance device to control the voltage disposition in a voltage divider circuit, such as is illustrated schematically and simply in Fig. 1.

As shown in Fig. 1, the voltage divider circuit includes two resistors, 2 and 3, connected in series and energized from an input circuit 4. An output circuiti is connected across the terminals of the resistor 3, and that circuit 5 is energized in accordance with the potential drop across the resistor 2.

If the two resistors, 2 and 3, are of fixed values, the voltage drop across each resistor will maintain the same proportion relative to the voltage drop across the other resistor, when the applied voltage I is varied.

Thus, for example where an electron tube is used to constitute the'resistor' 3 in the circuit in Fig. 1, the voltage dropacross the cathode-plate of the tube will have the same ratio to the applied voltage E from the input circuit 4, as the tube resistance will have to the total resistance of the voltage dividing circuit.

As indicated in Fig. 2, the cathode-plate resist-' ance of the tube varies with the grid excitation When an electron tube'is employed as aresistor element in the voltage dividing circuit, the circuit arrangement becomes such as is shown in the diagram oiFlg. 3. The input circuit 4 is shown connected to the voltage dividing circuit, which includes the resistor 2 and the triode 3a. through coupling condensers 8, and 9 of low im- The triode 3a is shown as a typical heater-type triode, in which the heater element is shown as energized from a source of heating energy, as a battery ll, connected to the heater element 2 through two R. F. chokes l2 and I3. The cathode is connected to one conductor of the input circuit 8 through the condenser 9. The grid is provided with a biasing potential derived from a source indicated as a battery I, that is connected to the grid through an R. F. choke 23.

In order to balance out the inter-electrode capacitance of the triode a parallel shunt, including an inductance l1 and a capacitance I8, is connected acrossthe tube 3a. The constants of the shunt elements l1 and l8-are such, that, at

the operating radio frequency, the tube capacity and the shunt are resonant to said frequency and constitute a non-reactive circuit, with the shunt resistance being very high compared to the tube plate-cathode resistance. The plate voltage for the triode is provided by'a source indicated as a battery it. The negative terminal of battery 19 is grounded, and the positive terminal is connected through'an R. F. choke 2| to the plate of the triode 3a. The cathode D. C. return path is provided by an R. F. choke 22. The grid biasing battery It is grounded at the positive terminal.

The plateof the tube is connected to the resistor 2 through a condenser l5 having negligible impedance at the operating radio frequency to prevent short circi'iiting of the plate battery ill by the impedanceelement ll of the parallel shunt.

-The'grid electrode of the triode'3a is shown provided with a by-pass condenser 25 to the cathode, to provide a low resistance path to any appreciable radio frequency voltage between the grid and the cathode while, at the same time, permitting the application of a direct current biasing voltage between the cathode and the grid.

The two condensers 8 and 9 in the input circuit, and two similar condensers 26 and 21 in the output circuit, connected to the terminals of the triode, are provided to isolate the direct current voltage within the voltage divider assembly or -Fig. 3, when that assembly is connected to the The ratio or the output voltage, representing the dro across the tube, to the input voltage, as applied to the voltage dividing circuit, depends upon the value 01 the resistor 2, and the relationship of that value to the resistance characteristics of the tube 3a.

Since the resistance of the tube in between the cathode and the anode, will vary with the excitation of the grid, the ratio oi the output voltage to the input voltage will also vary with the grid excitation.

One oithe primary objects of my invention is to provide a voltage dividing circuit including a triode or other form of electron tube, with such circuit characteristics that a voltage derived from the voltage divider shall be a linear function over a substantial region or the grid voltage applied to the tube.

In Fig. '4, several curves 20a to 28c are shown, representing the relationship between the ratio of output voltage to input voltage, and thecorresponding grid excitation, for different values oi. resistance in the series resistor2 connected to the tube In inFlg. 3.

The value 01' the resistor 2, corresponding as.

each curve 28a to 28c is shown at the upper end of each curve. The middle curve 28c corresponding to a resistance value of 15,000 ohms in series with the tube, shows a linear relationship between the ratio 01' output voltage to input voltage Eo/E and the direct current grid voltage, between values oi Eo/E=0.795 and Flo/ 0.47. Thus with a resistor 2 having a certain fixed value of resistance in series with the triode So, an output to input voltage ratio may be established that will bear a linear relationship to the grid voltage over a substantial range.

In-the present instance, in the circuit'shown in Fig. 3, the resistor 2 has a resistance value of 15,000 ohms, and the triode 3a is the tube known commercially as the 6C5." The middle curve 280 in Fig. 4 illustrates the linear relationship between the grid voltage and the ratio of the output voltage to the input voltage 01 the voltage divider circuit. According to that curve, as the grid voltage is varied between ten volts and sixteen volts, a direct linear relationship will be established between the grid voltage and the ratio of the voltage across the plate and cathode electrodes of the triode, to the input voltage, across .the circuit including the resistor 2 and the triode 3a in series.

Thus, with a circuit including a tube, and'a resistor having the proper resistance value corresponding to the characteristics of the tube, a direct linear voltage relationship may be established between the grid excitation oi the tube la and an output voltage of the network circuit over an extended range'oi the relationship, where the input voltage is held constant.

While in the above circuit a tube has been used as a non-linear resistance, it is evident that any other known type of element, which has a nonlinear relation between its resistance or impedance and the controllingiunction applied thereto, can be used. In addition, all or part of resistance 2 in Figures 1 and 3 can be constituted by the internal resistance or the source of voltage E.

By making the impressed voltage E, in Fig. 3,

a source 0! carrier potential ahd placing a source of modulating potential in series {with battery It, the output E0 will be an amplitude modulated carrier which will bear alinear relationship to the modulating voltage over a range \depending upon the constants of the circuit.

Tube controlled phase shifter x! It is well known that when an alternating volt? age is applied to a circuit including a resistor and an impedance device in series. the voltage drop across the impedance device is angularly displaced iron: the voltage drop across the resistor, andthat each such voltage drop is displaced from the ap- 7 plied voltage. Such circuit arrangement is shown, for example, in Fig. 5. in which the applied voltage i'rom an input circuit 4, similar to that of Fig. 2, is applied to a circuit including the resistor 3 and an impedance device 29. The circuit is similar to that shown in Fig. 2, except that the impedance device 28 is inductive or reactive,.as distinguished from the non-inductive resistor 2 in the circuit in Fig. 2. v

Fig. 6 illustrates a circuit similar to that shown in Fig. 3, except that device 29 connected in series with the triode la, is an impedance instead of the non-inductive resistor 2 of Fig. 3.

In Figs. 7 and 8, are shown vector voltage diagrams of the voltage dividing circuit shown in Fig. 5, depending upon whetherthe reactor 28 is inductive or capacitive. If the reactor 29 is inductive, the relationship between the applied voltage and the voltage drop across the reactor 29 and across'the resistor 3 will be as shown in Fig. 7. It the impedance 2! is purely capacitive, the relationship between the voltage drop across the reactor-29 and the voltage drop across the resistor 2 is reversed with respect to the applied voltage as shown 111' P18. 8.

as the drop of potential across the reactor 29, in

such manner as to vary the angle between the applied voltage and-the voltage across the resistor.

Another important purpose of the present invention is to provide a system wherein the phase angle of the output voltage with respect to the input voltage will be a linear function of the voltage applied to the grid of the triode 3a in'the system illustratively shown inFig. 6. The phase angle between the applied input voltage from circult 4, and the output voltage across the triode to will be the angle whose tangent is represented by the traction, of which the numerator is the potential drop across reactor 20 and the denominator is the potential or triode 3a, 01' Fig. 6.

In Fig. 9, several curves Illa to Mid are drawn. showing the relationship between the'grid excitation voltage, of and the phase angle between the applied voltage E and the output voltage E0, the latter corredrop across the resistor,

sponding to the potential drop across the triode.-

The curve 30b, corresponding to the case where a reactor is used having 15,000 ohms reactance. shows a linear relatio p between the grid voltage and the phase angle between the applied voltthe electron tube to in Fig. 6,

age and the output voltage for the range of values of the phase angle between 15 and 55. Thus, by use of proper circ t constants in a voltage dividing system such as shown in Fig. 6, a triode may be employed as a variable resistor, and an output voltage derived across the triode, whose angular relationship a linear function of the amplitude of the modulation voltage applied to the tube over a substantial range of that voltage.

It should be noted that considering amplitudes only, the variation of Eo/E with respect to variation in grid voltage is not a linear relationship. Curve 30a, in Fig. 9, shows the amplitude variation when reactance 29 has a .value of 15,000 ohms. If this is considered undesirable it can be eliminated by conventional amplitude limiting or automatic volume control methods.

As is the case with Figs. 1 and 3, any other type of non-linear impedance can be used instead of 'tube 3a in Figs. 5 and 6. Also all or part of reactance' 29 can be constituted by the internal rea stance of voltage source E.

If source E in Fig. 6 is a source of carrier voltage and a source of modulating voltage is inserted in series with grid battery H, the output E0 will be a phase modulated wave wherein the phase shift in degrees, will bear allnear relationship to the modulating voltage. The amplitude modulation can be eliminated by methods above referred to.

Although in the above described circuits the linear relationships are functions of the grid voltage, the circuits can be arranged to make said relationships a function of the voltage impressed on another electrode or combinations of electrodes.

Another phase modulation method In Figs. 10 to 13, inclusive, 1 have illustrated another arrangement for establishing phase modulation on a carrier voltage of high frequency.

Abasic diagram of the system is shown in Fig.

to the input voltage will be parallel to the first circuit and including a capacitor 63 and a resistor 64 in series. The two parallel circuits thus constituted are grounded to complete the parallel connection of the two circuits.

A voltage dividing circuit shown schematically as including two resistors 65 and 66, is connected between the two parallel circuits and is energized by the potential difference between the two terminal sections of the voltage dividing circuit, at

points 61 and 66. The juncture point 10 between the two resistors 65 and 66 of the voltage dividing circuit will have a potential difference relative to ground, that will depend upon the relationship between-the resistance values of the resistor 65 and the resistor 66.

If the resistor 66 be varied in accordance with some modulating voltage. the potential of the juncture point ill will be varied, and the potential difference between the juncture point I0 and ground will then provide the output voltage whose phase-angular relationship with respect to the initial source voltage may be caused to vary in accordance with the amplitude'of the modulating voltage.

' In Fig. 11 is shown a vector diagram of the various voltages as distributed in the network of the point C, corresponding to 'point 6'! below the inductor 6|, are on opposite sides of the applied voltage DE, and the potential difference between the points B and C represents thevoltage that is applied to the voltage-dividing circuit including the resistors-65 and 66. The potential of the juncture point A, representing, potential of point I0 between resistors 65 and 66, corresponds to the point of intersection between the applied voltage vector DE and thev potential diflerenee line 3-0. That point of intersection, indicated" as A, represents the point Ill of floating potential, that shifts above or below the line DE, on the line 3-0 as a locus. a

If the resistance of the resistor 66 were increased, with a resultant shift of the potential of the point 10 to the position indicated by the point A on the line BC, the output voltage of the modulating network would then be represented by broken line DA', and the amount of phase-displacement would be represented by the angle between the line DE and the broken line DA'.

In the basic circuit of the simple diagram shown in Fig. 11, the constants and values of the elements in the two parallel circuits are chosen so the current values in thetwo main branches will be relatively high compared to the. small cross-current that will flow through the voltage dividing circuit including the resistors 66 and 66 so that variation of the cross-current has a negligible effect on the fixed phase shifts produced by the phase shifting. elements. As the voltage drop across the variable resistor 66 is D -A, in Fig, 11, from the original solid line DA, up to about 30 degrees phase shift.

For the sake of illustration, the vector diagram in Fig. 11 has been shown symmetrical. In a circuit arrangement such as in Fig. 13, however, where the voltage dividing circuit includes a triode, it may be desirable to have the initial voltage line DA perpendicular to the locus line 3-0 at a point that is not at the middle of the line BC.

If the characteristics of the network are such, that at the quiescent point the juncture II is so located that the voltage represented by the line BA is not equal to the voltage represented by the line C--A, then the elements represented by voltage lines DB and DC should be so proportioned that the line B-C joining these two v to those in Fig. 10, and the elements in the volt-.

j ass-met of said capacitor and resistor, said third branch comprising an inTpedance conR age dividing circuit correspond to those already identified in Fig. 3.

The :triode"3a in Fig. 13corresponds to the;

resistor It, in'Fig. 10, and resistor 2 corresponds to resistor 65. -The combination of resistor 2 and tube 3a and the other elements shown are the same as similarly numbered elements in Fig. 3.

Choke 22 functions to keep the cathode at a high R. F. potential with respect to ground. Fig. 13 shows, in addition a modulating network for the grid voltage of 3a wherein voltage applied to lead 38 is coupled through condenser 53 across resistor ii in series with grid-biasing battery ll. Modulation voltage applied to leads 38 will vary the grid bias and, hence, the voltage drop in tube to. This voltage variation will result in phase modulation as above explained in connection with Figs. to 12. Condenser "has a high impedance to the modulating voltage but serves to by-pass any carrier voltage from the source of oscillations III. As the modulating voltage varies, the grid potential changes the resistance of the triode Ia, and the potential of the floating potential point It as in Fig. 13, iscorrespondingly shifted between the limits indicatedby the two thirty degree angles of the vector diagram of Fig. 12. The unsymmetrical arrangement of voltages illustrated is necessitated by the fact that the center of linear operation for the tube controlled voltage divider chosen occurs at a voltage ratio of 0.62.

A slight improvement in linearity at phase shifts greater than degrees can be obtained by using a voltage divider network having a slightly non-linear characteristic and setting the quiescent point so that the angle between the output vector and the vector for the controllable voltage across the controllable voltage divider is not a right angle. r

The phase modulators above described can be used as components of conventional frequency modulation sys Armstrong and others that pure frequency modulation is equivalent to phase modulation in which the'i'naximum' phase shift for a given modulating voltage is inversely proportional to the modulating frequency. Hence, any device which produces phase modulation will produce frequency modulation by making the modulating voltage inversely proportional to the modulating frequency. Ofcourse, at any single modulating frequency there is no distinction between the two. Hence, in this invention reference has been made to phase modulation with the understanding that by properly correcting the modulating voltage the same devices and methods will produce frequency modulation. In the claims the expression wavelength modulation" will be used as a generic designation of both phase and frequency modulation systeins.

The specific circuits and their constants above described are to be considered as illustrative of the principles of the invention. Numerous modifications can be made without departing from the spirit of the invention as set forth in the appended claims.

I claim: I

1. A phase 'modulation system for a source of carrier'voitage comprising a pair of branches connected in parallel across'said source, one branch comprising an inductor and a resistor in series, the other branch comprising a capacitor and a resistor in series, a third branch connected between the junction of said inductor and resistor and the Ju tion nected in series with the anode-cathode path of a grid-controlled electron tube,.means to vary the voltage on said grid over a predeterminedrange,

and an output circuit having one terminal connected to the Junction of said anode-cathode path and said impedance and a second terminal con- I nected to one side of said carrier voltage source, the resistance of said anode-cathode path over said range of grid voltage variation being a nonlinear function of voltage variation on said grid,

the magnitude of said impedance being so related to the average resistance of said-anode-cathode path over said range that the ratio'of the magnitude of a'given characteristic of the voltage across said anode-cathode path with respect to the magnitude of a like characteristic of the voltage across said third branch is a substantially linear function of said grid voltage variation.

2. A phase modulation system for a source of carrier voltage comprising a pair of branches connected in parallel across said source, one branch comprising an inductor and a resistor. in series, the other branch comprising a capacitor and a resistor inseries, a third branch connected between the junction of said inductor and resistor and the junction of said capacitor and resistor. said third branch comprising a resistor connected in series with the anode-cathode path of agridcontrolled electron tube, means to vary the voltage on said grid over a predetermined range, an output circuit having one terminal connected to the junction of said anode-cathode path and said resistor and a second terminal connected to one side of said carrier voltage source, the resistance of tems. It has been pointed out by said anode-cathode'path over said range'of grid voltage variation being a non-linear function of voltage variation on said grid, the resistance of said resistor being so related to the average resistance of said anode-cathode path over said range that the ratio of the voltage drop across said anode-cathode path with respect to the voltage across said third branch is a substantially linear function of said grid voltage variation.

. 3. A wave-length modulation system for a constant frequency carrier source comprising a pair of parallel branches connected across the terminals of said source, one branch includinga fixed inductance in series with a fixed resistance, the

'other branch including a fixed capacitance in wherein both resistances are equal, and the inductive reactance equals the capacitative reactance at the frequency of said source.

5. A'modulation system as set forth in claim 3, wherein said inductance and capacitance each have one terminal connected to each other.

6. A modulation system as set forth in claim 3,

I wherein the relative impedances of said branches are such that the current in the third branch is relatively small compared to the currents in the other branches. I

'1. A modulation system as set forth in claim 3.

an electron tube, and wherein said modulating means varies the impedance of said space-current path. V

8. A modulation system as set forth in claim 3, wherein said third branch comprises a fixed resistance in series withthe, anode-cathode path of an electron tube, wherein said modulating means varies the impedance of said path, and a parallel resonant circuit connected across said P th and tuned to the frequency of said source.

9. A substantially linear amplitude-modulation network for a source of carrier voltage comprising a pair of predominantly-resistive elements connected in series between the terminals of said source, one element being a resistance, the other.

element being the space-current path of an elec-' tron tube having control means to vary the resistance of said space-current path, a source of modulation voltage variable overa predetermined range, said control means being connected solely to said modulation-voltage source, an output circuit connected solely across said spacecurrent path, the resistance of said path being a non-linear function of said modulation voltage over the range of variation thereof, the magnitude of said resistance being so related to the average resistance ofsaid space-current path that the output-voltage amplitude is a substantially linear function of said modulation-voltage variation.

10. A substantially linear amplitude-modulation network for a source of carrier voltage comis high compared to the impedance of'said spaceprising a pair of predominantly-resistive elements connected in series between the terminals of said source, one element being a resistance, the other element being the space-current path of an elec- I tron tube having control means to vary the resistance of said space-current path, a source of modulation voltage variable over a predetermined range, said control means being connected solely to said'modulation-voltage source, an output circuit connected solely across one of said elements, the resistance of said path being a noncurrent path. v

12. A modulation network as set forth in claim 10, wherein said output circuit together with the capacity of said space-current path is parallelresonant to the frequency of said source.'

13. A'modulation network as set forth in claim 10, wherein the capacity. of said space-current path is neutralized.

EDWIN K. S'I'ODOLA. 

